The ${\mathrm{}}^{13}C(\alpha ,n){\mathrm{}}^{16}O$ reaction constitutes the dominant neutron source for the main $s$ process, which operates at a thermal energy of $\mathit{kT}=8$ keV. Since the cross section at stellar energies is very small, the reaction rate cannot be directly determined and has to be extrapolated from cross section results obtained at higher energies. To remove various discrepancies in the normalization of previous data sets and to subsequently improve the reliability of the extrapolation, we performed measurements of the ${\mathrm{}}^{13}C(\alpha ,n){\mathrm{}}^{16}O$ reaction in the energy range $E_{\mathrm{c}.\mathrm{m}.}=320\text{−}700$ keV. In addition, the double differential scattering cross section ${\mathrm{}}^{13}C(\alpha ,\alpha ){\mathrm{}}^{13}C$ was measured in the energy range $E_{\mathrm{lab}}=2.6\text{−}6.2$ MeV for 28 angles. These data were used to constrain possible contributions from background resonances for a reliable extrapolation with the multichannel $R$-matrix code SAMMY. As a result, the uncertainties were significantly reduced, and a reaction rate of $(4.6\pm 1.0)\times {10}^{-14}$ ${\mathrm{cm}}^3$/moles at $\mathit{kT}=8$ keV ($T=0.1\times {10}^9$ K) was determined.

Neutron capture nucleosynthesis in massive stars plays an important role in galactic chemical evolution as well as for the analysis of abundance patterns in very old metal-poor halo stars. The so-called weak $s$-process component, which is responsible for most of the $s$ abundances between Fe and Sr, turned out to be very sensitive to the stellar neutron capture cross sections in this mass region and, in particular, of isotopes near the seed distribution around Fe. Activation measurements in a quasistellar neutron spectrum corresponding to a thermal energy of $\mathit{kT}=25$ keV have been carried out on ${}^{58}\mathrm{Fe}$, ${}^{59}\mathrm{Co}$, ${}^{64}\mathrm{Ni}$, ${}^{63}\mathrm{Cu}$, and ${}^{65}\mathrm{Cu}$. By a series of repeated irradiations with different experimental conditions, uncertainties between 3.0% and 4.6% could be achieved, factors of 2 to 3 more accurate than previous data. Compared to previous measurements, severe discrepancies were found for ${}^{63,65}\mathrm{Cu}$. The consequences of these results have been studied by detailed model calculations for convective core He burning and convective shell C burning in massive stars.

The s-process branching at A (176) has been analyzed on the basis of significantly improved experimental cross sections. This work reports on activation measurements of the partial (n, gamma) cross section of Lu-176 feeding the isomeric state in Lu-176. In total, six irradiations were performed at the Karlsruhe 3.7 MV pulsed Van de Graaff accelerator, and the induced activities were measured with HPGe clover detectors. In combination with previous data, partial cross sections of 3185 +/- 156 and 1153 +/- 30 mbarn were deduced at kT = 5: 1 and 25 keV, respectively. With these results and a recent time-of-flight measurement of the total stellar (n, gamma) cross section, the isomeric ratio was found to be constant in the relevant thermal energy range of the main s-process component. Based on these new data, a comprehensive analysis of the branching at Lu-176 was carried out for testing the temperature and neutron density conditions during He shell flashes in thermally pulsing low-mass asymptotic giant branch stars. It was found that the long-standing problem of the mother/daughter ratio of the two s-only isotopes Lu-176 and Hf-176 could be solved, if the temperature-dependent beta-decay half- life of Lu-176 was considered with sufficient resolution over the temperature profile of the convective He shell flashes

The strengths of low-energy resonances in ${}^{14}\mathrm{N}(\alpha ,\gamma {)}^{18}\mathrm{F}$ at 573 keV and 1136 keV have been measured using an activation method. In addition, their relative strength and the energy of the lower resonance have been determined in a prompt $\gamma$-ray experiment. The results of these measurements are used to reevaluate the stellar reaction rate of ${}^{14}\mathrm{N}(\alpha ,\gamma {)}^{18}\mathrm{F}.$ The present reaction rate at temperatures of astrophysical interest is a factor of 2 smaller than previously reported.

The stellar ($n,\hspace{0.5em}\gamma$) cross sections of ${}^{79}\mathrm{Br}$, ${}^{81}\mathrm{Br}$, ${}^{85}\mathrm{Rb}$, and ${}^{87}\mathrm{Rb}$ have been determined by a series of ten activation measurements in a quasistellar neutron spectrum corresponding to a thermal energy of $\mathit{kT}\hspace{0.5em}=\hspace{0.5em}25$ keV. The final uncertainties between 3 and 10% were dominated by the γ-ray intensities in the decay of the respective reaction products. All other uncertainties were significantly reduced through variation of the experimental parameters. The consequences of these results for $s$-process nucleosynthesis of Br and Rb, and in particular of the local $s$-only isotopes ${}^{80,\hspace{0.5em}82}\mathrm{Kr}$ and ${}^{86,\hspace{0.5em}87}\mathrm{Sr}$, are discussed with respect to the branching points at ${}^{79}\mathrm{Se}$ and ${}^{85}\mathrm{Kr}$. The contributions of both the main $s$ component from thermally pulsing low mass asymptotic giant branch stars and the weak $s$ component from massive stars are considered in this analysis.

The stellar $\left(n,\gamma \right)$ cross sections of the Ne isotopes are important for a number of astrophysical quests, i.e., for the interpretation of abundance patterns in presolar material or with respect to the $s$-process neutron balance in red giant stars. This paper presents resonance studies of experimental data in the keV range, which had not been fully analyzed before. The analyses were carried out with the $\mathcal{R}$-matrix code sammy. With these results for the resonant part and by adding the components due to direct radiative capture, improved Maxwellian-averaged cross sections (MACS) could be determined. At $kT=30\mathrm{keV}$ thermal energy we obtain MACS values of $240\pm 29,1263\pm 160$, and $53.2\pm 2.7 \mu \mathrm{barn}$ for ${}^{20}\mathrm{Ne},{}^{21}\mathrm{Ne}$, and ${}^{22}\mathrm{Ne}$, respectively. In earlier work the stellar rates of ${}^{20}\mathrm{Ne}$ and ${}^{21}\mathrm{Ne}$ had been grossly overestimated. ${}^{22}\mathrm{Ne}$ and ${}^{20}\mathrm{Ne}$ are significant neutron poisons for the $s$ process in stars because their very small MACS values are compensated by their large abundances.